I 9 2 THE BOOK OF NATURE STUDY 



the shell, which seem to be formed by the deposition of extremely 

 delicate layers of lime around an entombed microscopic parasite 

 the larva of a Trematode worm. In fact, a pearl is the sepulchre 

 of a fluke. (5) Sometimes mussels come so near the bank that 

 they are caught by rooks, and the interesting sight has been seen of 

 rooks letting the shells fall from a height upon stones (perhaps 

 accidentally to begin with), so that the succulent mollusc became 

 available. These five items are merely illustrations of different 

 ways in which one may approach animals like these, and of course 

 there are as many more. What better illustration could one 

 have, for instance, of the interlinking of living creatures of the 

 web of life than in the fact that the young of the freshwater 

 mussel has to spend part of its youth as a parasite on freshwater 

 fishes, such as the stickleback, and that a carp-like fish of Central 

 Europe the bitterling or Rhodeus amarus lays its eggs in the 

 mantle-cavity of pond mussels. There they are fertilised by 

 inwafted sperms, and they develop for about a month inside the 

 gills. " The mollusc reciprocates by throwing off its embryos 

 on the parent fish, in the skin of which they remain encysted for 

 some time, the period of reproduction of the fish and mussel 

 coinciding.'* 1 The eggs of the freshwater mussels develop inside 

 the outer gill-plates ; the young feed there on nutritive slime ; 

 they become larvae known as Glochidia, with strong attaching 

 hooks on the margins of the bivalved shell, and with a long fila- 

 ment called the " byssus " cord. When fishes come near (or some- 

 times when they are absent) the mother mollusc liberates the 

 larvae, which are extremely sensitive to the presence of fish. Mr. 

 Latter 2 states that the " tail of a recently killed stickleback thrust 

 into a watch-glass containing Glochidium throws them all into 

 the wildest agitation for a few seconds ; the valves are violently 

 closed and again opened with astonishing rapidity for fifteen to 

 twenty-five seconds, and then the animals appear exhausted and 

 lie placid with widely gaping shells unless they chance to have 

 closed upon any object in the water (e.g. another Glochidium), 

 in which case the valves remain firmly closed. " If no host is 

 available the Glochidia die. We have digressed in regard to this 

 matter partly because it illustrates the sort of natural history 



1 See (Bridge and Boulenger) p. 222. 2 See (A. H. Cooke) p. 222. 



